The American Historical Association has spied itself a Problem with a capital P and it is determined to do something about it. That problem? Too many people are reading history doctoral dissertations on the Internet.
This madness must be stopped, the AHA thought to itself. We can't have all these people reading scholarly works online, for free. And so, the AHA crafted a solution, not a perfect one -- what solution is? -- but something that might help, something that might prevent all these people from reading all these dissertations. Yesterday, in a statement posted online, where everybody may read it (and many have), the AHA encouraged graduate programs and university libraries to please embargo completed dissertations from the Internet for six years.
Here's the AHA in its own words (emphasis added):

The American Historical Association strongly encourages graduate programs and university libraries to adopt a policy that allows the embargoing of completed history PhD dissertations in digital form for as many as six years. Because many universities no longer keep hard copies of dissertations deposited in their libraries, more and more institutions are requiring that all successfully defended dissertations be posted online, so that they are free and accessible to anyone who wants to read them. At the same time, however, an increasing number of university presses are reluctant to offer a publishing contract to newly minted PhDs whose dissertations have been freely available via online sources. Presumably, online readers will become familiar with an author's particular argument, methodology, and archival sources, and will feel no need to buy the book once it is available. As a result, students who must post their dissertations online immediately after they receive their degree can find themselves at a serious disadvantage in their effort to get their first book published; it is not unusual for an early-career historian to spend five or six years revising a dissertation and preparing the manuscript for submission to a press for consideration. During that period, the scholar typically builds on the raw material presented in the dissertation, refines the argument, and improves the presentation itself. Thus, although there is so close a relationship between the dissertation and the book that presses often consider them competitors, the book is the measure of scholarly competence used by tenure committees.
In the past, most dissertations were circulated through inter-library loan in the form of a hard copy or on microfilm for a fee. Either way, gaining access to a particular dissertation took time and special effort or, for microfilm, money. Now, more and more university libraries are archiving dissertations in digital form, dispensing with the paper form altogether. As a result, an increasing number of graduate programs have begun requiring the digital filing of a dissertation. Because no physical copy is available, making the digital one accessible becomes the only option. However, online dissertations that are free and immediately accessible make possible a form of distribution that publishers consider too widespread to make revised publication in book form viable.